The present invention relates to a system for transferring sheets in a printing machine. More particularly this invention concerns a device for picking up sheets from a stack or the like and advancing these sheets to a continuously rotating receiver drum.
In order to feed a succession of sheets through a printing machine it is normally necessary to pick the sheets of a stack or otherwise displace them away from a supply location where they are either standing still or moving at a relatively low speed, to accelerate the sheets up to the peripheral speed of a first receiver drum, and to space the sheets apart in the direction of flow through the press by the desired distance. It is essential for good printing that the sheets pass in an orderly succession through the machine.
This is typically done, as shown in German Pat. No. 92,731 by means of a rotary transfer device which picks the sheets up from a pick-up station and passes them at a transfer station to the receiver drum or element of the printing machine. The receiver drum normally rotates as the same uniform speed and is provided on its outer surface with receiver grippers that grasp or otherwise engage the leading or trailing edge of a sheet fed to it at the transfer station. The transfer device, however, normally is provided with at least two angularly spaced grippers, which term is intended here to include a simple clamping arrangement, suction holders, and the like, and is rotated in steps, normally by a Maltese-cross arrangement. Thus the rotary transfer device comes to a complete halt or slows down very greatly as each of the grippers is aligned with the pick-up station so that the gripper can grasp the edge of a sheet, and then accelerates very rapidly so that by the time the gripper holding a sheet is aligned with the transfer station the sheet is moving at the same speed as the gripper of the receiver. When the two grippers of the transfer device are spaced apart by 180.degree. the path between pick-up station and transfer station is normally slightly less than 180.degree. so that immediately after a sheet is transferred from one gripper to the receiver the rotary transfer device can be arrested as the other gripper picks up the next sheet to be fed.
Such an arrangement therefore requires extremely rapid acceleration and deceleration of the transfer mechanism, resulting in extremely rapid wear of the mechanism thereof. Furthermore appropriate timing of the various devices in this arrangement becomes an extremely complex matter due to the very rapid acceleration and deceleration.